Adaptive modulation scheme and data rate control method

ABSTRACT

A radio communication system includes a first radio station for dividing codewords into communication units and transmitting the divided codewords by modulating them in every communication unit, and a second radio station for coupling and decoding signals obtained by demodulating the communication units. The radio stations have common information of first bits, equal to maximum bits per symbol of the communication units, and an encoding type list. The first station modulates and transmits the communication units with a modulation type with second bits. The second radio station receives the communication units modulated by the first radio station, demodulates the communication units with a modulation type with third bits, combines and decodes the demodulated signals with encoding types in the encoding type list, and obtains a result of the decoding, as reception information, by an encoding type in which no error is detected in the result of decoding.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 11/028,938, filedJan. 5, 2005 (now U.S. Pat. No. 7,801,236). This application relates toand claims priority from Japanese Patent Application No. 2004-061578,filed on Mar. 5, 2004. The entirety of the contents and subject matterof all of the above is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates in general, to a radio communicationsystem; and, more particularly, the invention relates to a modulationtype and data rate control method which can adapt to variations in thequality in each propagation path in a radio communication system usingmultiple propagation paths, such as provided in a multi-carrier radiocommunication system.

BACKGROUND OF THE INVENTION

In view of the increasing data rate per time in the conventional radiocommunication system, a multi-level modulation technique for propagatinginformation of multiple bits per symbol has been developed. In such amulti-level modulation technique, the maximum throughput increases, whenthe quality of the propagation paths is further improved, as the numberof bits per symbol increases. On the other hand, the throughput largelydecreases, when the quality of the propagation paths is lowered, as thenumber of bits per symbol increases. Therefore, an adaptive modulationtechnique has been proposed to switch the modulation levels according tothe quality of the propagation paths so as to realize stablecommunication. This technique has been described in the paper titled“Transmission Characteristic of Modulation Level Variable AdaptiveModulation Type” (B-II Vol. J78-B-II No. 6 pp. 435-444, June 1995 of TheTransactions of the Institute of Electronics, Information andCommunication Engineers)

(Non-Patent Document 1).

Moreover, with the frequency band of radio communication expanding, theOFDM (Orthogonal Frequency Division Multiplexing) system fortransmitting information by dividing a transmission into many orthogonalsubcarriers has been used, and the OFDM adaptive modulation type systemto switch the modulation type for each subcarrier has also been proposedto cover a difference in the quality of the propagation paths within theOFDM bandwidth. This technique has been described in the paper titled“OFDM Adaptive Modulation type utilizing Multilevel Transmission PowerControl for High-Speed Data Communication” (B-II Vol. J84-B-II No. 7 pp.1141-1150, July 2001, of The Transactions of the Institute ofElectronics, Information and Communication Engineers) (Non-patentdocument 2).

Patent Document 1] “Transmission Characteristic of Modulation LevelVariable Adaptive Modulation type” (B-II Vol. J78-II No. 6 pp. 435-444,June, 1995, of The Transactions of the Institute of Electronics,Information and Communication Engineers) by Otsuki, et al.

[Patent Document 2] “OFDM Adaptive Modulation type utilizing MultilevelTransmission Power Control for High-Speed Data Communication” (B-II Vol.J84-B-II No. 7 pp. 1141-1150, July, 2001, of The Transactions of theInstitute of Electronics, Information and Communication Engineers) byYoshiki, et al.

Both a transmitter station and a receiver station need to have commoninformation on a modulation type in order for the receiver station toperform correct demodulation during switching of the modulation typeaccording to a variation in the propagation path by the conventionaladaptive modulation technique.

If the transmitter station and the receiver station have a differentmodulation type, the data location is deviated and continuous errorsoccur. Therefore, in the scheme, for example, where the modulation typeis communicated from a transmitter station to a receiver station, thesignal carrying this information must always be sent with a higheraccuracy. Moreover, even in the case where the receiver station assumes,for example, the modulation type from the received signal, a trainingsignal for highly accurate estimation is necessary.

Therefore, when it is attempted to control the modulation type for eachsubcarrier in the OFDM system, a problem arises, in that the throughputof the data signal itself is suppressed because the signals forcontrolling the modulation type, such as the modulation type, indicatingsignal and the training signal, increase. On the contrary, when thenumber of signals for controlling the modulation type, such as themodulation type indicating signal and the training signal, is reduced soas to not suppress the throughput of the data signal, a problem alsoarises in that the degree of freedom in the modulation type control isreduced, fine control becomes impossible, and communication by fullyutilizing the propagation path cannot be realized.

SUMMARY OF THE INVENTION

The present invention has been conceived and developed to solve theproblems described above. In the case where the modulation type iscontrolled for each subcarrier in the OFDM, for example, communicationcan be realized even if the modulation type is not always matchedprecisely between the transmitter station and receiver station.Therefore, an object of the present invention is to provide a radiocommunication system in which the modulation type can be switched foreach subcarrier, while the number of control signals for controlling themodulation type is reduced, and the throughput of the communication as awhole can be controlled according to the total quality of thepropagation path.

As a means for solving the problems described above, in the adaptivemodulation and encoding type system of the present invention, thetransmitter station and receiver station have, in common, the maximumtransmission bits per symbol in each subcarrier and list information ofthe encoding type to be selected.

The transmitter station selects the modulation type in the subcarrierfrom the propagation path quality of each subcarrier, performs channelencoding by selecting the encoding type from the above-stated list basedon the bit number information which may be used for the communication inthe selected modulation type, and divides the code in units of themaximum transmission bit number per symbol to each subcarrier. In eachsubcarrier, only the bits that can be transmitted in the selectedmodulation type among the distributed bits are modulated fortransmission.

The receiver station performs demodulation by selecting the modulationtype used for demodulation in the subcarrier from the quality ofpropagation paths of each subcarrier, summarizes the result ofdemodulation with addition of the signal suggesting reception of thesignal in the degree of likeliness of 0 when the number of bits obtainedby the demodulation per symbol is less than the maximum bits per symbol,executes, to this demodulation result, on trial, the demodulation in theencoding type listed in the channel encoding type list, and determinesthe transmission of the information obtained as a result of demodulationwhen the demodulation is completed successfully.

With the method described above, the modulation type can be switched foreach subcarrier according to the propagation path without attemptingaccurate matching of the modulation type between the transmitter stationand the receiver station. Moreover, the throughput can be controlledaccording to the average quality of the propagation paths.

According to the present invention, a system is provided in which thenumber of bits to be propagated per symbol can be switched according tovariation of the propagation path without communication of themodulation type information between the transmitter station and receiverstation, and the optimum throughput can also be selected forcommunication according to variation in the propagation paths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of adaptive modulation andencoding employed in a transmitter station in accordance with thepresent invention;

FIG. 2 is a diagram showing an example of adaptive demodulation anddecoding employed in a receiver station in accordance with the presentinvention;

FIG. 3 is a graph showing the relationship between the number of bitsfor communication through adaptive modulation and the channel encodingrate in accordance with the present invention;

FIG. 4 is a graph showing the relationship between the number of bitsfor communication through adaptive modulation and the transmission powerin accordance with the present invention;

FIG. 5 is a functional block diagram showing a first embodiment of thetransmitter station in accordance with the present invention;

FIG. 6 is a functional block diagram showing a second embodiment of thetransmitter station in accordance with the present invention;

FIG. 7 is a functional block diagram showing a first embodiment of thereceiver station in accordance with the present invention;

FIG. 8 is a functional block diagram showing a second embodiment of thereceiver station in accordance with the present invention;

FIG. 9 is a functional block diagram showing an embodiment of anadaptive modulator of the transmitter station in the case where thepresent invention is adapted to a system for transmitting information byemploying multiple codes;

FIG. 10 is a functional block diagram showing an embodiment of theadaptive modulator of the transmitter station in the case where thepresent invention is adapted to a system for transmitting information byemploying multiple antennas;

FIG. 11 is a schematic diagram showing an embodiment of the adaptivemodulator of the transmitter station in the case where the presentinvention is adapted to a system for transmitting information byemploying multiple beam patterns;

FIG. 12 is a flow diagram showing an example of process flow in thetransmitter station of the present invention; and

FIG. 13 is a flow diagram showing an example of process flow in thereceiver station of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described withreference to the accompanying drawings. However, as an example, themaximum number of bits per symbol of each subcarrier is set to 6 bitsand as a modulation type, an adaptive modulation type using 64 QAM or 16QAM or QPSK has been used. However, it should be understood that thepresent invention is not limited to such maximum bits and modulationtype. Namely, the present invention can be adapted in general to asystem in which the maximum bits per symbol is set to 2 m bits anddemodulation is conducted with the 2^(2k)QAM (k is a natural numberequal to or less than m) modulation type. Moreover, the 4QAM modulationtype, in which k=1, indicates a modulation type similar to QPSK.

First, an example of the configuration of the transmitter station andthe receiver station and the flow of signals will be described on thebasis of the structural diagrams of the transmitter station and receiverstation, using the adaptive modulation and channel encoding types of thepresent invention, as illustrated in FIG. 5 to FIG. 8.

However, in the following description, only the structure used foradapting the modulation and demodulation types, in accordance with thepresent invention, to a communication from the transmitter station tothe receiver station will be described. Actually, however, it is alsopossible in accordance with the present invention to carry out two-waycommunication by providing both the functions of the transmitter stationand the functions of the receiver station to only one communicationapparatus. Moreover, the transmitter station and receiver station inaccordance with the present invention are used for a radio station forconducting modulation of a data signal and a radio station forconducting demodulation. Any one of these stations may be defined as abase station and a mobile terminal station. Moreover, when theseterminal stations are mutually connected, any of the terminal stationsmay be used as a transmitter station and a receiver station.

FIG. 5 illustrates a first example of the structure of a transmitterstation in accordance with the present invention. FIG. 6 illustrates asecond example of the structure of a transmitter station in accordancewith the present invention. FIG. 7 illustrates a first example of thestructure of a receiver station in accordance with the presentinvention. FIG. 8 illustrates a second example of the structure of areceiver station in accordance with the present invention.

On the occasion of making communication with the transmitter station ofFIG. 5 or FIG. 6 and the receiver station of FIG. 7 or FIG. 8, thetransmitter station and receiver station have in common the maximum bitsper symbol of each subcarrier and the list information for selecting thechannel encoding type, and this information is stored prior to thechannel encoding of transmitted information and decoding of the receivedsignal.

As a method of having common information in the transmitter station andreceiver station, a certain scheme may be defined as part of the system,or it may also be communicated from the transmitter station or receiverstation at the time of starting a communication. Moreover, in the casewhere any transmitter station or receiver station is defined as a basestation and the other is designated as a terminal station, it ispossible to introduce a method in which a notice is transmitted to theterminal station from the base station when the location is registeredand hand-over is conducted, or a method may be employed in which thereference signal to be broadcasted from the base station is transmittedwith inclusion of the common signal. Moreover, the number of maximumbits per symbol is determined on the basis of the condition of thepropagation path, the amount of requests for communication and theperformance of transmitter and receiver stations.

In the transmitter station of FIG. 5, an adaptive modulation controller410 converts the signal received by an RF module 400 into a signal of afrequency domain with FFT (Fast Fourier Transformation), selects themodulation type based on the propagation path quality of eachsubcarrier, communicates respective modulation type information of eachsubcarrier to an adaptive modulator 430, and also communicates the totalnumber of bits per symbol of the modulation type selected for eachsubcarrier to a channel encoder 420.

The channel encoder 420 selects the encoding type from the encoding typelist based on the information obtained from the adaptive modulationcontroller 410, conducts the channel encoding based on the selectedencoding type, and transfers the result of channel encoding to theadaptive modulator 430. In the channel encoding, error detectioninformation is added to the information to be propagated using a method,such as parity and CRC; the channel encoding is conducted using, forexample, the channel code, such as the convolutional code and Turbocode; the bit position in the generated codeword is varied with theinterleave; and the number of bits distributed to each subcarrierincludes a remainder, part of the bits being deleted with puncture.

The adaptive modulator 430 divides the data transferred from the channelencoder 420 into subcarriers, conducts the modulation with themodulation type selected by the adaptive modulation controller 410, andmultiplexes each subcarrier signal into signals of a time domain usingthe IFFT (Inverse Fast Fourier Transformation) for transmission throughthe RF module 400.

The signal propagated from the transmitter station of FIG. 5 isreceived, for example, by a receiver station having the structureillustrated in FIG. 7. In the receiver station of FIG. 7, an adaptivemodulation controller 510, using the FFT, converts the signal receivedby an RF module 500 into signals of the frequency domain; selects themodulation type used for demodulation on the basis of the propagationpath quality of each subcarrier; communicates the respective modulationtype information used for demodulation of each subcarrier to an adaptivedemodulator 530; and communicates the total number of bits per symbol ofthe modulation type used for demodulation selected for each subcarrierto a channel decoder 520.

The adaptive demodulator 530 converts the signal received by the RFmodule 500 to signals in the frequency domain with the FFT; demodulatesthe signal using the modulation type indicated by the adaptivedemodulation controller for each subcarrier; assumes the reception ofthe signal having a degree of likeliness of 0 as much as the shortagewhen the number of bits per symbol of the modulation type used fordemodulation is less than the maximum number of bits per symboldetermined previously for the transmitter station; outputs the result ofdemodulation of the maximum number of bits per symbol for each symbolfrom each subcarrier; and then transfers all multiplexed carriers to achannel decoder.

The channel decoder 520 selects the encoding type from the encoding typelist, performs depuncture, deinterleave of the data transferred from theadaptive demodulator 530 according to the selected encoding type and thedecoding of the channel code, and determines whether the decoding resultis erroneous or not using the error detection information added at thetime of transmission.

When the decoding result is determined to be not erroneous, the channeldecoder 520 outputs the determined decoding result as the receivedsignal. When the decoding result is determined to be erroneous, anotherencoding type is selected from the preceding encoding type list andsimilar decoding is attempted again. Moreover, when the decoding resultis determined to be erroneous for all encoding types in the list, anindication of failure of decoding is outputted.

In the trial operation of the channel decoder 520, as the type fordetermining the selection sequence from the list of the encoding type tobe used for the decoding, it is possible to previously determine asimple sequence of selection before starting the communication to alwaysselect a predetermined sequence. As another type, it is also possiblethat the reception quality is communicated from the adaptivedemodulation controller 510, the encoding type of a higher channelencoding rate is attempted with priority for the trial when thereception quality is high, and the encoding type of a lower channelencoding rate is selected with priority when the reception quality islow. As yet another type, it is also possible to select and attempt withpriority, an encoding type which has succeeded in the decoding used inthe preceding decoding operation. This encoding type is suitable whenthe quality change in the propagation path is comparatively gradual. Inaddition, as still another type of encoding, it is also possible tocommunicate the encoding type selected by the transmitter station forthe channel encoder 420 to the receiver station, in response to whichthe receiver station selects and performs on trial with priority theencoding type indicated by the transmitter station.

The receiver station in accordance with the present invention may alsobe realized with a structure such as that shown in FIG. 8, in place ofthe structure shown in FIG. 7. In the receiver station of FIG. 7, theadaptive demodulation controller 510 and adaptive demodulator 530execute processes similar to those of the adaptive demodulationcontroller 510 and adaptive demodulator 530 in the receiver station ofthe structure of FIG. 6. The channel decoder 520 of the receiver stationof FIG. 8 is provided with depuncture, deinterleave, propagation pathdecoding and error detecting functions for each list of the encodingtype and is also capable of outputting, as received information, anoutput of the channel decoder which has been proven to be not erroneousas a result of the error detection process.

The structure of the transmitter station in accordance with the presentinvention as illustrated in FIG. 5 may be replaced, for example, withthe structure of FIG. 6. In the transmitter station in the structure ofFIG. 6, the adaptive modulation controller 410 measures the totalquality of the propagation path from the signal received by the RFmodule 400 and the result to the channel sends encoder 420.

The channel encoder 420 selects the encoding type from the encoding typelist on the basis of the information obtained from the adaptivemodulation controller 410, executes the encoding based on the selectedencoding type, and transfers the result to the adaptive modulator 430.In the encoding process, the error detection information, such as paritybits or CRC bits, is added to the information to be transmitted.Moreover, the information is encoded with the channel code, such as theconvolutional code and Turbo code. The bit location in the generatedcodeword is altered with the interleaving, and if any bits remain afterit has been distributed to each subcarrier, the bits are punctured.

The adaptive modulator 430 divides the data transferred from the channelencoder 420 to each subcarrier, executes the modulation using themodulation type for propagating the maximum bits per symbol asdetermined for the receiver station in all subcarriers, and transmitsthe data to the RF module 400 through multiplexing of each subcarriersignal into signals in the time domain using IFFT (Inverse Fast FourierTransformation).

In the transmitter station having the structure of FIG. 6, switching ofthe modulation type according to the propagation path quality of eachsubcarrier is not performed in comparison with the transfer station havethe structure of FIG. 5.

However, since the receiver station of FIG. 7 and FIG. 8 switches themodulation type used for demodulation according to the propagation pathquality of each subcarrier, it is possible to achieve the advantage thatthe structure of the transmitter station can be simplified, while thecommunication is realized by utilizing the difference in the quality ofthe propagation paths of each subcarrier.

Moreover, the present invention is also applicable even when it isdifficult to measure the difference in the propagation path quality ofeach subcarrier in the transfer station, while the frequency, forexample, used for communication to the receiver station from thetransmitter station is different from that used for communication to thetransmitter station from the receiver station by using the transmitterstation having the structure of FIG. 6. In this case, it is preferablethat the reception quality is received as feedback information from thereceiver station.

Next, a detail description will be presented below concerning themodulation type and encoding type in accordance with the presentinvention in the transmitter station and receiver station with referenceto FIG. 1 and FIG. 2.

The transmitter station and receiver station have in common the maximumbits Nc per symbol of each subcarrier and information consisting of aselectable encoding type list. In the following description andaccompanying figures, a value expressed as Nc indicates the maximum bitsper symbol of each subcarrier.

The adaptive modulation controller, channel encoder, and adaptivemodulator in the transmitter station will be described in detail withreference to FIG. 1. In the transmitter station, the adaptive modulationcontroller 110 selects the modulation type 114 of each subcarrier on thebasis of the propagation path quality 111 of each subcarrier and obtainsthe number of bits Nb 115 used for communication per symbol of theselected modulation type. A value expressed by Nb in the followingdescription and figures indicates the number of bits 115 used forcommunication per symbol selected at the transmitter station side.

Here, for selection of the modulation type of each subcarrier, it ispossible, for example, to use signal power intensity, interference powerintensity, signal to interference and noise power ratio or the like asthe propagation path quality.

Moreover, in regard to selection of a threshold value, the thresholdvalue 112 is simply selected from the point of view of selecting themodulation type in which the number of modulation levels is higher thanthat of QPSK when the propagation path capacity of the subcarrierassumed from the propagation path quality 111 of each subcarrier islarger than 2, which is the number of bits used for communication persymbol of QPSK. Moreover, the threshold value 113 is selected from thepoint of view of selecting the modulation type in which the number ofmodulation levels is higher than that of 16QAM when the propagation pathcapacity is larger than 4, which is the number of bits used forcommunication per symbol of 16QAM. When it is further required toimprove the communication quality, the threshold value can be selectedaccording to the code, modulation type, and propagation path quality.

The channel encoder 120 performs the encoding by selecting only oneencoding type from the encoding type list provided in common for thetransmitter station and receiver station. An example of the criterionfor selecting the encoding type is illustrated as the relationship 301indicated with the graph of FIG. 3.

In the relationship 301, the encoding type is selected, in which thechannel coding rate is smaller than the value obtained by dividing thetotal sum of the number of bits Nb used for communication per symbol inthe modulation type selected for each subcarrier in the section totransmit one codeword with the total sum of the maximum bits Nc persymbol of each subcarrier.

In the example of FIG. 1, for example, one code is transmitted at a timeusing six subcarriers. Therefore, the encoding type is selected so thata code length 122 becomes equal to a value obtained by adding themaximum bits Nc per symbol of each subcarrier for six subcarriers and aninformation bit length 121 becomes smaller than a value obtained byadding the number of bits Nb used for communication per symbol of themodulation type selected by each subcarrier for six subcarriers. Here,the selected encoding type is sufficient when it satisfies the conditionthat the channel encoding rate is smaller than the quotient between thetotal sum of Nb and total sum of Nc. Moreover, for example, in the casewhere the code is transmitted by division into n codes in the timedomain, the encoding type is selected so that the code length becomesequal to the product of the total sum of Nc for all carriers used fortransmission of codes and n, and the information bit length of this codebecomes smaller than product of the total sum of Nb of all carriers usedfor transmission of codes and n.

Owing to the selection of such encoding type, as described above, thedata rate can be improved because the encoding type which assures ahigher channel coding rate when the propagation path quality is higheris selected, the error correction capability is improved because theencoding type which assures a lower channel encoding rate when thepropagation path quality is lower is selected, and the encoding typedepending on the propagation path quality is also selected.

In the relationship 301 of FIG. 3, a constant channel encoding rate isselected, for example, within the range 302. However, energy used perbit of the transmitting information can be stabilized and thecommunication quality can also be improved by controlling the power sothat the transmission power becomes larger as the quotient between thetotal sum of Nb and the total sum of Nc becomes small in the section 312to select the identical channel encoding rate, as seen in the diagram ofFIG. 4, which shows the relationship between the quotient of the totalsum of Nb and total sum Nc and the transmission power.

In the code selection scheme described above, the channel encoding rateis selected with reference to the quotient of the total sum of Nb andtotal sum of Nc. However, it is also possible to introduce thepropagation path quality as the selection criterion of the channelencoding rate. A similar effect can also be attained with the encodingtype selecting method. For example, it is possible that, under thecondition that the value obtained by multiplying a coefficient having apositive correlation with the error correction capability of the channelcode to a value obtained by dividing the total sum of the communicationpath capacity of the propagation path in the section to transmit onecodeword with the total sum of Nc is defined as the reference value, thetype where the channel encoding rate is nearest to the reference valueamong the encoding type list is selected, or the type where the channelencoding rate is largest within the range not exceeding the referencevalue among the encoding type list is selected, or the type where thechannel coding rate is smallest within the range not exceeding thereference value is selected.

The adaptive modulator 130 divides the codeword generated by the channelencoder 120 into each subcarrier in a unit of the maximum bits Nc persymbol of each subcarrier, and it generates a transmission signal bymodulating the signal with the modulation type of subcarrier using thenumber of bits Nb used for communication per symbol of the modulationtype of subcarrier selected by the adaptive modulation controller 110among the Nc bits distributed for each subcarrier. In this case, thebits not used for transmission as many as the difference between the Ncbits and Nb bits are generated; however, these bits are not used fortransmission, but not wasted.

Next, the adaptive demodulation controller and decoder of the receiverstation and processes in the demodulator will be described in detailwith reference to FIG. 2. In the receiver station, the adaptivedemodulation controller 210 selects the modulation type 214 to be usedfor demodulation of each carrier from the propagation path quality 211of each subcarrier and obtains the number of bits Nd 215 to be used forcommunication per symbol of the selected modulation type. In thefollowing description and figures, a value expressed by Nd indicates thenumber of bits used for communication per symbol selected in thetransmitter station side.

Here, for selection of the modulation type of each subcarrier, forexample, signal power intensity, interference power intensity, signal tointerference and noise power ratio, etc. may be used as a measure of thepropagation path quality, as in the case of the transmitter station.

Moreover, the threshold value can be selected so that, for example, thethreshold value is selected to 212 to select the modulation type in amodulation level higher than QPSK when the propagation path capacity ofthe subcarrier assumed from the propagation path quality 211 of eachsubcarrier is larger than 2, which is the number of bits used forcommunication per symbol of QPSK. In addition, the threshold value isselected to 213 to select the modulation type in a modulation levelhigher than 16QAM when the capacity is larger than 4, which is thenumber of bits used for communication per symbol of 16QAM. When furtherimprovement in the quality of propagation paths is required, thethreshold value may be selected according to the code used, themodulation type and the propagation path characteristic. Moreover, thecommunication quality can also be improved by setting the thresholdvalues 212, 213 in the receiver station to values smaller than thethreshold values 112, 113 in the transmitter station.

It is most desirable that the propagation path quality 211 measured withthe receiver station is matched with the propagation path quality 111measured with the transmitter station (indicated with a dotted line inthe figure), but these are not always matched due to a difference in thephysical environment of the transmitter station and the receiver stationand the timing of the measurement. As a result, in the adaptivemodulation and channel encoding type processing of the presentinvention, the modulation type 214 selected in the receiver station isnot always matched with the modulation type 114 selected fortransmission in the transmitter station. However, in accordance with thepresent invention, since the maximum bits Nc per symbol of eachsubcarrier is fixed without relation to the modulation types 214 and114, an error in the selection of the modulation type is never relatedto an error in the modulation result of the other subcarrier. Moreover,since the error correction capability of the code is used, thecharacteristics are never deteriorated to a large extent even whendifferent modulation types are selected in the transmitter station andreceiver station. Therefore, when a code having a sufficient errorcorrection capability is used, a means for attaining the accuratematching of the modulation type in the transmitter station and receiverstation is no longer required, unlike the conventional technique.

The adaptive demodulator 230 executes the demodulation process with themodulation type used for demodulation of each subcarrier selected by theadaptive demodulation controller 210 to obtain the signal of Nd bits,adds the signal under the assumption that the signal having a degree oflikeliness of 0 is received as much as the difference between the Ncbits and Nd bits, and then outputs the signal of Nc bits in total foreach subcarrier to the decoder 220.

The decoder 220 executes, on trial, the decoding by selecting one ormultiple channel encoding types from the encoding type list provided incommon for the transmitter station and completes the demodulation of thetransmission information under the assumption that the data is encodedin the transmitter station with the type succeeded in the demodulation.Determination for successful demodulation can be made when no error canbe detected in the demodulation process, for example, even after thedetection signal, such as parity and CRC, is added at the time oftransmission.

When the demodulations by multiple channel encoding types are attemptedfor trial with the channel decoder 220, calculations can be saved, forexample, with the method for estimating the encoding type from Nd and Ncusing a method similar to the method for selecting the encoding methodin the transmitter station and then executing on trial with priority theestimated method and with the method for executing on trial with thepriority the encoding type succeeded in the previous decoding.

As a result of the channel decoding process described above, thedecoding result and error detection signal are sent to a hostcommunication layer from the channel decoder 220 as the reception resultof the receiver station of the present invention.

In regard to the adaptive modulation and channel encoding in accordancewith the present invention, the process flow in the transfer station isillustrated in FIG. 12, while the process flow in the receiver stationis illustrated in FIG. 13.

In the process flow shown in FIG. 12, the transmitter station is firstselects the maximum bits Nc per symbol of each subcarrier with theprocess 600. The process 600 is sufficient when it is executed in aseries of communication units, and this process is also executed withthe interval for which this process is set once when the communicationis started.

Next, the transmitter station measures the propagation path quality inthe process 601, selects the modulation type of each subcarrier in theprocess 602, based on the result of measurement, and selects the channelencoding type in the process 603. These processes are performed with theinterval following the variation rate of the propagation path. Namely,when the variation rate of the propagation path is identical to the codelength, the type of processes 602 and 603 is selected on the basis ofthe result of measurement in the process 601 under the interval of abouteach transmission of code. When variation of the propagation path israther lower in comparison with the code length, the process isperformed with the lower interval. Next, the transmission information isencoded in the process 604 in the transmitter station in every code andthe encoded information is transmitted through modulation with theprocess 605 in unit of symbol.

In the process flow shown in FIG. 13, the receiver station firstselects, in the process 610, the maximum bits Nc per symbol of eachsubcarrier. The process 610 is sufficient when it is executed in everyseries of communication units like the process 600 in the transmitterstation with the interval for which it is executed once when thecommunication is started. Moreover, the process 510 must be executed inthe same timing as the process 600 in the transmitter station.

Next, the receiver station measures the propagation path quality in theprocess 611 and selects the modulation type used for demodulation ofeach subcarrier in the process 612 on the basis of the result ofmeasurement. These processes are executed in the interval following thevariation rate of the propagation path. When the variation rate of thepropagation path is identical to the code length, the type in theprocess 612 is selected based on the result of measurement in theprocess 611 with the interval similar to the code length of thereceiving signal. When the variation of propagation path is rather lowerthan the code length, this process is executed in the lower interval.The interval of this process is desirably similar to that of theprocesses 601, 602 and 603, but it is not always required to be matchedtherewith.

Next, the receiver station demodulates the received signal based on themodulation type selected in the process 612, estimates the channelencoding type in the process 613 for every completion of thedemodulation of the code length, and executes the decoding process inthe process 614. The processes 613 and 614 are executed once for everycompletion of the demodulation of the code length when the receiverstation includes multiple decoders like the receiver station of FIG. 8.Meanwhile, when the receiver station uses only one decoder through theswitching operations like the receiver station of FIG. 7, it executes ontrial the decoding with the candidate type for every completion of thedemodulation of the code length. If decoding fails, the decoding isexecuted on trial with a different type, and when the decoding hassucceeded, the process is completed.

The modulation type has been described above for 2^(2m)QAM. However,application of the present invention is not limited to 2^(2m)QAM. Forexample, the present invention can be adapted to the a modulation typein which signal point allocation can be resettably expanded, forexample, using the Gray code, and assignment of bits to the signal pointcan be set. A similar control can also be adapted to 2^(m)PSK and2^(m)ASK by defining the maximum bits per symbol as m bits.

Moreover, the discussion of code and encoding appearing in the abovedescription does not suggest only encoding by use of the errorcorrection code, such as a simple convolutional encoding and Turboencoding, but more generally indicates the mapping to the bit train formodulation from the information as the communication object. Forexample, processes including he signal processes for addition of anerror detection code, interleave, repetition and puncture are calledencoding.

In above description, moreover, the number of bits called the bits persymbol or the maximum bits per symbol corresponds to the number of bitsof a codeword after the encoding, and this number of bits can be made tocorrespond to the number of transmission information bits by multiplyingit by an encoding rate of the code.

Moreover, in above description, communication is performed by dividingthe information into multiple subcarriers using IFFT and FFT operationsin propagation paths which are different in quality for every frequencyused. However, the present invention can be generally applied to asystem for effecting communication by dividing the information intoplural communication units of different quality.

For example, the adaptive modulator may take the form illustrated inFIG. 9. It is also possible in accordance with the present inventionthat, in the adaptive modulator 430 of FIG. 9, the data is spread, usinga code like the Walsh code in place of performing a IFFT, and isdespread in the adaptive demodulator 530 in place of a FFT, and thecodeword is divided into multiple codes.

Moreover, the present invention can also be applied in a manner suchthat, in place of performing IFFT in the adaptive modulator 430, thesignal is divided as illustrated in FIG. 10 and the modulated signalsare transmitted from different antennas; and, moreover, in place ofperforming FFT in the adaptive demodulator 530, the codeword is dividedfor multiple antennas for transmission by executing an interferenceeliminating process to isolate the signals received with multipleantennas into respective signals for each transmitting antenna.

The present invention can also be applied in such a manner that, inplace of performing IFFT in the adaptive modulator 430, the signal isdivided, for example, as illustrated in FIG. 11 and the signals aretransmitted from different antennas after the primary conversion bymultiplying a complex coefficient to the modulated signals; and,moreover, in place of performing FFT in the adaptive demodulator 530,the signals received with multiple antennas are subjected to theinterference eliminating process to isolate the signals before primaryconversion at the transmitter station in order to realize division ofthe codeword into multiple beam patterns.

1. A radio communication system comprising: a first radio station; and a second radio station; wherein the first and second radio stations have common information of a first number of bits which is equal to a maximum number of bits per a communication unit, and an encoding type list including encoding types and code lengths after encoding; wherein the first radio station is configured to: generate codewords by encoding transmission information, generate a plurality of parts of codewords by picking up a first number of bits from the codewords, transmit at least one communication unit by picking up a second number of bits which is equal to or less than the first number of bits from the parts of codewords; and wherein the second radio station is configured to: receive and demodulate at least one communication unit transmitted by the first radio station, and combine and decode the demodulated signals of the at least one communication unit, including adding a third number of signals each corresponding to a smallest likelihood, the third number of signals also corresponding to the difference of the first number of bits and the second number of bits.
 2. A radio communication system according to claim 1, wherein the communication unit corresponds to a plurality of modulation symbols.
 3. A radio communication system according to claim 1, wherein the communication unit corresponds to a plurality of subcarriers.
 4. A transmitter station, for transmitting information to a receiver station in a radio communication system, comprising: a memory means for storing information of a first number of bits as a maximum number of bits per communication unit, and an encoding type list including encoding types and code lengths after encoding, which is used for encoding the transmission information; a channel encoding means for generating codewords by encoding the transmission information with one of the encoding types listed in the encoding type list, and for generating a plurality of parts of codewords by picking up the first number of bits from the codewords; and a transmitting means for transmitting one or a plurality of communication units, at least in part by picking up from the parts of codewords a second number of bits which is equal to or less than the first number of bits.
 5. A transmitter station according to claim 4, wherein the communication unit correspond to a plurality of modulation symbols.
 6. A transmitter station according to claim 4, wherein the communication unit corresponds to a plurality of subcarriers.
 7. A transmitter station, for transmitting information to a receiver station in a radio communication system, comprising: a memory configured to store information of a first number of bits as a maximum number of bits per communication unit, and an encoding type list including encoding types and code lengths after encoding, which is used for encoding the transmission information; a channel encoder configured to generate codewords by encoding the transmission information with one of the encoding types listed in the encoding type list, and to generate a plurality of parts of codewords by picking up the first number of bits from the codewords; and a transmitter configured to transmit at least one communication unit at least in part by picking up from the parts of codewords a second number of bits which is equal to or less than the first number of bits.
 8. A transmitter station according to claim 7, wherein the communication unit correspond to a plurality of modulation symbols.
 9. A transmitter station according to claim 7, wherein the communication unit corresponds to a plurality of subcarriers. 